Bonding different types of bulk material together to form a bonded article has many applications in a variety of industries. For example, in the semiconductor industry the ability to bond an insulator material to a conductive material is highly desired. Moreover, being able to attach an insulative material, such as sapphire, to a metal, like titanium, has many applications including the formation of packaging for photonic and medical devices. Another example industry that desires the ability to effectively attach sapphire to titanium is the manufacturing of high end jewelry. This method also has applicability for bonding sapphire windows in high vacuum systems, and military and space vehicles.
Traditional methods of bonding these types of material together use high temperature and high pressure to create a diffusion bond that is typically in the tens to hundreds of microns thickness range. The temperatures used can be in the order of 600-1000° C. These high temperatures can result in harm to the products they are forming. For example, when forming packages for devices, high temperatures used to form the bonds in the package risk damaging components that are within the package. This also results in grain growth in polycrystalline materials and alters their physical properties. Moreover, undesirable compounds form in bond regions with the use of high temperatures over long formation times that can affect the strength or toughness of the bond. Additionally, differences in the thermal expansion coefficients of the materials bonded using diffusion bonding techniques can lead to the generation of interfacial stresses as the assembly cools from high temperatures. These stresses often result in crack generation. The size of the cracks that are created due to interfacial stresses tends to scale with the size of the interaction zone of the materials being joined. Since typical diffusion bonding creates realtively large interaction zones, the cracks that are generated also tend to extend several microns to millimeters. Brittle materials will spontaneously fail catastrophically even under small loads if the cracks exceed the critical flaw size. So larger cracks will likely result in reduced reliability and service life. For this reason, traditional diffusion bonding techniques have had limited success in creating mechanically robust bonds between materials with dissimilar thermal expansion coefficients. Other known methods for bonding titanium and sapphire include metallizing the sapphire with a thin film of niobium and brazing the components together with gold or other suitable braze material. Typical temperatures are 450 to 1200 C.
A few relatively low temperature methods used to bond dissimilar materials with the hope of not harming the internal components of a package utilize an intermediate layer, such as with solder, glass frit, or thermocompression bonding. These processes use moderate temperatures, ranging from 100° to 400° C., and moderate pressures to achieve the bond joint. As an example, solders of tin-indium-silver, or tin-lead, tin-copper-silver can be used. However, this approach has several disadvantages. The flux used for soldering is prone to cause contamination which is difficult to detect especially if the bond is used to seal the enclosure. The strength of the bond is inferior to that of the bulk materials, and these solder alloys are susceptible to corrosion if exposed to harsh environments. Another example is a thermal compression bond effected with the use of nano-porous gold and the application of high pressure to the bulk materials. However, it is difficult with these techniques to determine the integrity of the bond and the bond strength is far inferior to the strength of the bulk materials. Other methods using adhesives are used to form bonds however, the bonds are inherently weaker and non-hermetic.
For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a method of creating a bond between insulator and conductive bulk materials that is formed at low temperature and has a strength as strong as the bulk materials.